We have no time rules because I have never felt our kids routinely devote too much time to iPads. Of course some days they use them more than others, but they are also likely to go 2-3 days without using them at all. No one complains when I say it's time to go outside to play or when I pull out art supplies. I think if you put minimal effort into taking your children to the library, coloring with them, going outside with them etc....it would be odd for them to have so much free unstructured time that they could have an unhealthy attachment to tech time.